As higher recording density is required in magnetic recording, the surfaces of magnetic layer has been made smoother, and, therefore, a coefficient of friction in running between the magnetic layer and the parts of recording/reproducing device increases. As a result, it becomes marked problems that magnetic recording media are not smoothly run and that magnetic layers are damaged.
Recently, video tape recorders, personal computers and word processors have been commercially available and a magnetic recording medium for such uses has been used under various conditions, particularly in wide variety of temperature and humidity. In a rotary recording medium such as floppy disk which is used for personal computers and word processors, access and recess of the magnetic layer to the magnetic recording head are repeated many times. The running durability of the magnetic recording medium is big problem, particularly in such conditions that the recording medium is used at high temperature or between high temperature and low temperature for long time.
In order to solve the above problems in a so-called "coated type" magnetic recording medium comprising a magnetic layer containing ferromagnetic particles and a binder, a method for adding a fatty acid ester to a magnetic layer as described in JP-A-50-153905, JP-A-50-22603, JP-A-55-139637 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), JP-B-39-28367, JP-B-41-18065, and JP-B-47-12950 (the term "JP-B" as used herein refers to an "examined Japanese patent publication"); a method for adding a silicone compound to a magnetic layer as described in U.S. Pat. No. 2,654,681; and a method for adding a fatty acid or hydrocarbon to a magnetic layer have been proposed.
However, in these conventional methods, the above additives are easily vaporized from the surface of magnetic layer at high temperature, and deposited on the surface of magnetic layer at low temperature to cause head clogging and to increase dropouts. On the other hand, if an amount of the additives added is increased, the strength of magnetic layer is weakened because the binder in the magnetic layer is plasticized, and the durability of the magnetic layer is decreased.
Further, if an ester compound having a straight chain alkyl group, which has superior lubricating property is used, it is often deposited on the surface of the magnetic layer at low temperature, because it has a high melting point.
In order to resolve the above problems, it is disclosed in JP-B-47-12950, JP-A-58-160425, JP-A-58-218038, JP-A-60-205827, JP-A-61-294637, JP-A-62-117141 and JP-A-62-125529 that a fatty acid ester having a high molecular and a branched hydrocarbon group or an unsaturated hydrocarbon group is added to the magnetic recording layer, so that it hardly vaporizes from the layer at high temperature and it is not deposited on the surface of the magnetic layer at low temperature. However, no sufficient results have been obtained by these esters which are liquid at normal temperature, because the binder is plasticized by the esters having its miscibility with the binder, whereby the strength of the layer is decreased. And no sufficient lubricating property is provided by the ester compounds having an alkyl group.
Particularly, the defects caused by the ester compounds are serious problems in disk-form magnetic recording medium such as floppy disk which is used in such system that access of magnetic head in personal computers or word processors to magnetic layer of the medium is repeated many times, because the strength of the magnetic layer is decreased (i.e., tape durability is decreased and sufficient record/reproduction is not accomplished).
The problems of conventional methods as described above are more acute, as the surface of magnetic recording medium is more smooth. For example, in the coated type magnetic recording medium, when the polymer having a polar group capable of accelerating dispersion of the ferromagnetic particles is used as the binder as disclosed in JP-A-57-133521, JP-A-57-44227, JP-A-60-238309 and JP-A-61-104329, the problems are more serious because surface roughness of the magnetic layer becomes not more than 0.01 .mu.m.
Accordingly, the preparation of the magnetic recording medium having the magnetic layer with an excellent surface roughness and not having the above problems has been desired.
As binders used for the magnetic recording medium, it has been proposed in U.S. Pat. Nos. 4,409,299, 4,431,700, 4,510,203, 4,429,017, 4,439,486 and 4,568,613 to use thermoplastic resins such as a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer, a cellulose resin, an acetal resin or a urethane resin, alone or in combination. However, these resins do not provide the magnetic layer with sufficient abrasion resistance and, therefore, do not satisfy the requirement of magnetic recording medium having better running durability. On the other hand, it has been proposed in U.S. Pat. Nos. 3,597,273, 4,154,895, 4,409,291, 4,587,170, 4,049,871, 4,336,308, 4,411,956, 4,784,907, 4,068,040, 4,333,988 and 4,431,721 to use thermosetting resins such as a melamine resin or a urea resin, and to add crosslinkable binders such as an isocyanate compound or an epoxy ring-containing compound to the thermoplastic resins as described above. These resins provide the magnetic layer with better abrasion resistance and comparatively improved running durability. Further, it is proposed in the above references to introduce polar groups such as --COOH, --SO.sub.3 M or --PO.sub.3 M.sub.2 in the molecule in order to accelerate dispersion of ferromagnetic particles, whereby magnetic recording medium having excellent electromagnetic properties is obtained. However, the use of the crosslinkable binders provides such disadvantage that uniform physical properties of ferromagnetic particles as coating composition are not kept because of worse storage stability of a ferromagnetic particle dispersion, and therefore, uniform characteristics of magnetic recording medium are not maintained, and that a thermal processing step is required to harden the coated magnetic layer after coating and drying the coating composition. As the binder which is not accompanied by the disadvantages, a method for using oligomer or monomer of acrylic acid ester and hardening it by radiation exposure after drying it is disclosed in JP-B-47-12423 and JP-A-47-13639, JP-A-47-150104, JP-A-50-77433 and JP-A-56-25231. According to the method, a magnetic recording medium having excellent abrasion resistance can be obtained without having the procedural disadvantages as described above. However, a magnetic recording medium having both better electromagnetic properties suitable for high recording density and better running durability is not sufficiently obtained by the method.